In current communication networks, the subscribers or, more precisely, communication terminals assigned to said subscribers—such as, for example network termination equipment (NT: Network Termination)—are connected to central switching facilities or digital multiplexer devices (DSLAM: Digital Subscriber Line Access Multiplexer) via subscriber lines. The communication terminals on the subscriber side or the switching facilities are connected to the respective subscriber line—which can be embodied, for example, as a two-wire or four-wire line—are connected in each case via a subscriber line unit or via a modem arranged in the subscriber line unit, whereby a plurality of subscriber lines can be connected to one subscriber line unit.
Data communication between the modems via the subscriber line can take place, for example, using an xDSL transmission method. With transmission methods of this kind, one or more parameters characterizing the respective subscriber line (such as, for example, line length, terminating impedance, position of bridged taps) are determined by the modem on the switching center side as part of a prequalification process before the start of the actual data transmission and before a subscriber-side modem is connected (single-ended operation). In this way information relating to the maximum attainable data rate can be made available in advance in each case to the subscriber to be connected or the future user. In addition to the prequalification process, further methods for single-ended and double-ended testing of subscriber lines are known. Thus, for example, an ADSL prequalification method with echo canceller optimization to maximum selectivity is described in DE 10154937.